1. Generally speaking how long before an asphalt shingle roof should be replaced or does it simply depend on the appearance of the roof?
My home is 20 years old with the original shingles and except for a little moss (question about cleaning is next) it looks fine.
2. Have any of you used the Defy product, Roof Cleaner? Doesn't contain bleach, user friendly, etc.
Your roof can last another ten years provided it remains undisturbed. Aside from Mother Nature, worst thing for a roof? - walking on it; second worst thing - cleaning it (exception: cement-tile roofs can be safely cleaned with high pressure water only).